1. Field of the Invention
The present invention relates to a method and compositions for enhancing the rheology performance, shear recovery, and stability in viscoelastic surfactant fluids useful for treating formations in oil and gas-wells. More particularly the invention relates to viscoelastic surfactant fluids containing an enhanced shear recovery agent. The enhanced shear recovery agent comprises at least a sugar derived surfactant, a copolymer, or a combination thereof.
2. Description of the Related Art
Viscoelastic surfactant fluids have continued to grow in use in oilfield applications because of their advantages over conventional polymer systems. Such advantages include higher permeability in the oil-bearing zone, lower formation or subterranean damage, higher viscosifier recovery after fracturing, elimination of need for enzymes or oxidizers to break down viscosity, and easier hydration and faster build-up to optimum viscosity.
Viscoelastic surfactant fluids are useful in conventional hydraulic fracturing methods. Useful methods disclosed in U.S. Pat. No. 5,551,516 (Norman et. al.) is incorporated herein by reference. Oilfield applications and methods are described in “Oilfield Applications”, Encyclopedia of Polymer Science and Engineering, vol. 10, pp. 328-366 (John Wiley & Sons, Inc. New York, N.Y., 1987) and references cited therein, which are also incorporated herein by reference.
Hydraulic fracturing is a term that has been applied to a variety of methods used to stimulate the production of fluids such as oil, natural gas, from subterranean formations. In hydraulic fracturing, a fracturing fluid, usually an aqueous fluid, is injected through a well bore and against the face of the formation at a pressure and flow rate at least sufficient to overcome the overburden pressure and to initiate and/or extend a fracture(s) into the formation. The fracturing fluid usually carries a propant such as 20-40 mesh sand, bauxite, glass beads, etc., suspended in the fracturing fluid and transported into a fracture. The propant keeps the formation from closing back down upon itself when the pressure is released. The propant filled fractures provide permeable channels through which the formation fluids can flow to the well bore and thereafter are withdrawn. When the viscoelastic surfactant fluid is used as a fracturing fluid it may optionally contain a gas such as air, nitrogen or carbon dioxide to provide an energized fluid or foam. Supercritical carbon dioxide emulsion can be also present. In addition, other conventional constituents that perform specific desired functions, e.g., corrosion inhibitors, fluid-loss additives and the like can be present.
Growth in the use of viscoelastic surfactant fluids has been inhibited by the high cost of surfactants required to formulate such fluids. Another problem with use of viscoelastic surfactant fluids is their low shear recovery when subjected to high shear in treatment of subterranean formations. Furthermore, problems persist with use of viscoelastic surfactant fluids because of their low tolerance of organic/inorganic salts and clay stabilizers, such as potassium chloride (KCl) and tetramethyl ammonium chloride (TMAC), in subterranean formations. In addition, high temperatures encountered, especially up to 250° F., hinder the operation of deep well oilfield applications. These temperatures can break down the viscosity of viscoelastic surfactant fluids and render them ineffective in fracturing operations when viscoelastic surfactants are present at low concentrations in the viscoelastic surfactant fluid. High viscoelastic surfactant concentrations are used to avoid such viscosity breakdown. However use of viscoelastic surfactants at low concentrations in viscoelastic surfactant fluids results in unacceptably long shear recovery times. These unacceptably long shear recovery times hinder the operation of deep well oilfield applications.
In the prior art, attempts have been made to remedy breakdown in viscosity and low shear recovery performance by adding polymers, such as low molecular weight anionic polymers. Polymeric thickeners, e.g. starches, which thicken by entanglement of polymeric chains, have been used to viscosify the aqueous phase of suspensions. Such thickeners can degrade under the influence of mechanical shear or chemical scission (e.g. by oxidation or hydrolysis) of the polymeric chains resulting in loss of viscosity and suspension stability. Still the shear recovery can be unacceptably long and/or organic/inorganic salt tolerance may be inadequate.
In U.S. Patent Application Publication No. 2003/0134751 to Lee et. al., the addition of polymers improved the shear recovery of viscoelastic surfactant fluids. In particular, polymers with higher molecular weights than 25,000 were found to be effective at high concentrations of viscoelastic surfactant fluid. However, at low concentration, these polymers are not effective in sheer recovery.
U.S. Pat. No. 6,258,859 to Dahanayake et. al, discloses viscoelastic surfactant fluids containing amphoteric and zwitterionic surfactants, but the use of sugar derived surfactants, polymers, or combinations thereof for improvement of shear recovery time is not disclosed.
In U.S. Pat. No. 6,767,869 to DiLullo et. al., amphoteric polymers with high molecular weight, 500,000 or above, is added to viscoelastic surfactant fluids in well fracturing fluids. Use of sugar derived surfactants is not apparent from the above invention.
Accordingly, it would be desirable to have a surfactant additive that could be formulated on a cost-effective basis with a viscoelastic surfactant fluid, i.e., with relatively low levels of viscoelastic surfactant to be used in the treatment of subterranean formation in oil and gas wells. Furthermore it is desirable to have good shear recovery comparable to aqueous fluids used in fracturing which contain a relatively high concentration of viscoelastic surfactants. It would further be desirable to have a viscoelastic surfactant fluid that exhibits high tolerance with respect to organic/inorganic salts and clay stabilizers. It would also be desirable to have a single packaged viscoelastic surfactant fluid that could meet various temperature conditions.